With continual scaling of dimensions in semiconductor devices and increase in the number of gates per chip, yield management in semiconductor manufacturing has become critical for economical and profitable operation of chip manufacturing facilities. Referring to FIG. 1, past trends up to 65 nm technology node and forecast for future technology nodes show three components of yield loss including random defect yield loss, process limited yield loss, and parametric yield loss.
Random defect yield loss is the yield loss due to random defects generated during manufacturing of a semiconductor chip. As feature sizes shrink in a semiconductor chip, functionality of the semiconductor chip is more readily disturbed by random defects generated during semiconductor processing steps, resulting in increased in the random defect yield loss in successive technology generation. While the impact of random defects on yield becomes more severe in successive technology generations, semiconductor chip manufacturing facility automation and process enhancements tend to reduce generation of random defects and mitigate the increase in the random defect yield loss.
Process limited yield loss is the yield loss due to failure to control process parameters within specification. Process limited yield loss is thus due to an out-of-specification process parameter such as a thickness of a film, recess depth of a structure in a semiconductor substrate, and composition of a material. Inherent variability in semiconductor manufacturing processes causes some semiconductor chips to be out-of-specification for at least one process parameter. If the out of spec condition results in a short or open due then the chip is said to suffer from Process limited yield. Note, not all out of spec process parameters result in process limited yield. Increase in complexity of processing steps contributes to increase in the process limited yield loss, while improvement in process control tends to mitigate the impact of the processing steps on the process limited yield.
Parametric yield loss is the yield loss due to the fraction of manufactured semiconductor chips that do not meet performance specifications among the chips that do not suffer from random defect yield loss or process limited yield loss. In other words, the chips affected by the parametric yield loss do not have random defects or out-of-specification process parameters that result in a hard failure, such as a short or an open. If an out of spec process parameter does not result in a hard failure, such as a short or an open, but does contribute to a deviation from modeled simulated behavior, and this deviation contributes to the chip not meeting the performance specification, then the chip is said to suffer from parametric or circuit limited yield. Additionally, at least one of design specification for the chip, which may be, for example, circuit timing or power consumption in on-state or in off-state, is out of specification. The cause of the failure of the chip to meet the design specification may not be attributed to the random defect yield loss or process limited yield loss, but is attributed to statistical distribution of performance of individual semiconductor devices in the semiconductor chip. The fraction of the number of chips that do not meet the device specification due to these reasons relative to the number of chips that are not affected by random defect yield loss or process limited yield loss is the parametric yield loss, which is sometimes also referred to as “circuit limited yield loss.” Thus parametric yield loss has both a random, statistical component and a systematic component.
The impact of parametric yield loss increases in each succeeding technology node since more devices are integrated into a semiconductor chip. Recognizing the severe impact of the parametric yield loss, “design for manufacturability” (DFM) has been promoted as a concept. In essence, designers factor in potential yield impact of a particular design. While being a useful concept, design for manufacturability does not provide algorithms or methodology for systematically increasing parametric yield. Instead, it is a general recommendation to avoid designing circuits that may potentially cause performance problems.
It should be recognized, however, that employing an aggressive design that may potentially cause parametric yield issues is necessary to design a high performance chip. The difficult part is to estimate the balance between a potential gain in performance of an aggressive design and increase in parametric yield, i.e., decrease in parametric yield loss, of a conservative design.
Referring to FIG. 2, an exemplary prior art method for designing a semiconductor chip is shown in a flow chart 200. Referring to step 210, functional requirements of a chip are defined. The chip may be a processor, a volatile or non-volatile memory chip, or a system-on-chip (SoC) having multiple embedded components. Functional requirements include the nature of the chip as well as performance goals of the chip.
Referring to step 220, an electronic system level (ESL) description is generated based on the functional requirements of the chip. Electronic system level description and verification is a design methodology that focuses on the higher abstraction level without regard to lower level implementation. The goal of the ESL description is to enhance the probability of a successful implementation of functionality. Appropriate abstractions are utilized to generate a global level understanding of the chip to be designed. To this end, a high level programming language such as C or C++ is employed as an abstract modeling language to model the behavior of the entire system to be contained in a chip. Typically, this process is manual, although automation of this step by electronic design automaton (EDA) has been under investigation.
Referring to step 230, a register transfer level (RTL) description is generated from the electronic system level (ESL) description in the next chip design phase. Register transfer level (RTL) description is a description of a semiconductor chip design in terms of its operation. Specifically, the behavior of a circuit is defined in terms of data transfer, or flow of signals between hardware registers in the RTL description. Logical operations are performed on the data. A hardware description language (HDL) such as Verilog™ or VHDL™ is employed to create high-level representations of a circuit, from which lower level representations and ultimately actual discrete devices and wiring may be derived.
Referring to step 240, logic synthesis is performed to convert the RTL description in the form of the hardware description language (HDL) into a gate level description of the chip by a logic synthesis tool. Specifically, the gate level description is a discrete netlist of logic gate primitives, or “Boolean logic primitives.”
Referring to step 250, placement and routing tools utilize the results of the logic synthesis to create a physical layout for the chip. Logic gates and other device components of the netlist are placed in a “layout,” or a chip design. The chip design is then routed, i.e., wires are added to the placed components to provide interconnection between the components' signal and power terminals. Typically, this process is performed with tools employing electronic design automation (EDA) features.
Referring to step 260, power analysis and timing analysis is performed. It is noted that the exemplary prior art method scales power generation by scaling of a nominal device or multiple nominal devices. In other words, only the device type and device size are employed in the power analysis. The power analysis and the timing analysis are performed to check the chip design for functionality.
Referring to step 270, the chip design is analyzed to extract design specification. For example, timing analysis may be employed at this point to specify timing delay and expected chip operating frequency. Further, nominal leakage currents are estimated to specify power consumption of the chip.
Referring to FIG. 3, an exemplary prior art semiconductor chip manufacturing sequence including the steps of chip design is shown in a flow chart 300. Referring to step 310, a semiconductor chip design is provided as described in steps 210-260 of the flow chart 200 in FIG. 2. Referring to step 312, design specification is generated for the chip as in step 270 of the flow chart 200 in FIG. 2.
Referring to step 320, data preparation is performed on the chip design to generate various mask levels, which may then be transmitted to a “mask house,” or a mask fabrication facility to initiate fabrication of physical masks to be employed in manufacturing of semiconductor chips. Various “design comps,” or compensations to instances in design levels may be performed as part of data preparation. The mask house manufactures physical masks that may be subsequently employed in lithographic tools according to the mask level designs.
Referring to step 330, semiconductor chips are manufactured in a semiconductor chip fabrication facility. Typically, the semiconductor chips are manufactured on a semiconductor substrate such as a silicon substrate. Various semiconductor processing steps including lithography, deposition, and etching are employed.
Referring to step 340, the manufactured semiconductor chips are tested and characterized for functionality. Dysfunctional chips are sorted out. Operating frequency, on-state leakage, and off-state leakage are measured on functional chips.
Referring to step 350, parametric yield, i.e., circuit limited yield (CLY), is calculated for the group of semiconductor chips that do not suffer from random defect yield loss or process limited yield loss. Assuming a normal scenario in which the random defect yield loss and the process limited yield loss of the manufacturing process are within expected ranges, delivery of sufficient number of chips to a customer depends on the parametric yield loss. If the parametric yield exceeds a minimum parametric yield target value, sufficient number of chips meeting the design specification may be shipped to a customer, as shown in the step 360. If the parametric yield is below a minimum target value, not enough chips meeting the design specification are available for shipping, as shown in step 379.
In this case, few courses of systematic action are available to the semiconductor chip manufacturing facility for investigation of the source of the depressed parametric yield. While some methods are known in the art for diagnosing depressed process limited yield such as sorting the semiconductor chips by processing history or process variations, depressed parametric yield is much more difficult to investigate since the depressed parametric yield is correlated to specific design features of the semiconductor chip.
In financial perspective, when a semiconductor chip manufacturing facility commits to manufacture semiconductor chips based on an unknown chip design, the level of parametric yield loss is unpredictable from the perspective of the semiconductor chip manufacturing facility, while a customer generating a new chip design may have a vague idea of the level of expected parametric yield. Neither party has a good understanding on what level of parametric yield should be expected on the new chip design. Thus, lack of precise estimation of the parametric yield on the new chip design exposes a semiconductor chip manufacturing facility to a financial uncertainty, while the customer submitting the new design is not provided with any guidance on how to improve the design to enhance the parametric yield.
In view of the above, there exists a need for a system and methods for analyzing and managing parametric yield on a semiconductor chip during a chip design phase.
Further, there exists a need for a system and methods for analyzing and managing parametric yield on a semiconductor chip during a chip manufacturing phase or after a depressed parametric yield is observed in testing.
Yet further, there exists a need for a system and methods for predicting parametric yield at various levels to compare with observed data so that any anomaly in design may be found during testing of manufactured semiconductor chips.